AMINO-ACID CATABOLISM 21 



Experiments with isotopically labelled glycine have shown 

 that most of the methylene carbon appeared as acetic acid 

 whilst the carboxyl carbon appeared as CO 2 [4]. 



Alanine, serine or threonine serve as sole sources of carbon 

 and energy for CI. propionicum and are fermented to CO 2 , 

 NH3 and fatty acids [11, 12]: 

 3CH3CH(NH2)COOH+2HoO = 



2CH3CH,COOH+CH3COOH+CO,+3NH3 

 3CHo(OH)CH(NH.,)COOH+H.,0 = 



CH3CH2C6OH+2CH3COOH+2CO0+3NH3 

 The mechanism of alanine fermentation may be comparable 

 with that proposed for the Stickland reaction, alanine acting 

 both as the H-acceptor and as the H-donor. Lactate and 

 pyruvate are fermented in a similar manner and these fer- 

 mentations are at least superficially comparable to the fer- 

 mentation of lactate by the propionibacteria. However, 

 whilst in the latter propionic acid arises by the decarboxyla- 

 tion of succinate, in CI. propionicum it is probably formed 

 from acrylate [35]. Barker and Wiken have concluded that 

 acetate is not an intermediate in the fermentation of threo- 

 nine to butyric and propionic acids, and that butyric acid 

 probably arises directly from a C4-compound (a-ketobu- 

 tyrate?)[5]. 



Unlike D. glycinophilus and CI. propionicum, CI. tetano- 



morphum ferments glucose as well as certain amino-acids. 



The end-products of both glutamic acid and histidine 



fermentations include Hg , CO2 , NH3 , acetic and butyric 



acids [66], and by analogy with Edlbacher's work with liver, 



Woods and Clifton were the first to suggest that glutamic 



acid was an intermediate in the fermentation of histidine. 



Confirmation of this hypothesis has been recently obtained 



and the first step in the conversion of histidine (I) to 



glutamic acid, HCOOH and NH3 involves deamination to 



urocanic acid (II) [60]. 



CH=CH.CH2CH(NH2)COOH 



h ^i > 



N3 iNH -NH3 



CH 



(I) 



